diff --git a/ChangeLog b/ChangeLog
index 477fa06ee6..4ce8b085f0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sat Feb 16 18:25:14 2008  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* string.c (str_strlen): little more optimize.
+	  (rb_enc_nth): remove needless variable 'c'.
+
 Sat Feb 16 18:00:13 2008  Tanaka Akira  <akr@fsij.org>
 
 	* encoding.c (rb_enc_compatible): empty strings are always compatible.
diff --git a/string.c b/string.c
index 23f1d0addc..e6670d31ba 100644
--- a/string.c
+++ b/string.c
@@ -643,9 +643,11 @@ str_strlen(VALUE str, rb_encoding *enc)
 	    }
 	    while (s < t) {
 		unsigned long d = *s;
-		d = (~d ^ (d&(d<<1)))&NONASCII_MASK;
-		d = (d>>7) + (d>>15);
-		d = d + (d>>16);
+		d = ~d | (d<<1);
+		d &= NONASCII_MASK;
+		d >>= 7;
+		d += (d>>8);
+		d += (d>>16);
 #if NONASCII_MASK == 0x8080808080808080UL
 		d = d + (d>>32);
 #endif
@@ -657,6 +659,7 @@ str_strlen(VALUE str, rb_encoding *enc)
 	for (; p<e; p++) {
 	    if (((*p)&0xC0) != 0x80) len++;
 	}
+	return len;
     }
     else
 #endif
@@ -664,6 +667,9 @@ str_strlen(VALUE str, rb_encoding *enc)
     if (len < 0) {
 	rb_raise(rb_eArgError, "invalid mbstring sequence");
     }
+    if (ENC_CODERANGE(str) != ENC_CODERANGE_VALID && enc == STR_ENC_GET(str)) {
+	ENC_CODERANGE_SET(str, ENC_CODERANGE_VALID);
+    }
     return len;
 }
 
@@ -949,8 +955,6 @@ rb_str_s_try_convert(VALUE dummy, VALUE str)
 char*
 rb_enc_nth(const char *p, const char *e, int nth, rb_encoding *enc)
 {
-    int c;
-
     if (rb_enc_mbmaxlen(enc) == 1) {
         p += nth;
     }
@@ -981,10 +985,8 @@ rb_enc_nth(const char *p, const char *e, int nth, rb_encoding *enc)
         return (char *)p;
     }
     else {
-        for (c=0; p<e && nth--; c++) {
-            int n = rb_enc_mbclen(p, e, enc);
-
-            p += n;
+        while (p<e && nth--) {
+            p += rb_enc_mbclen(p, e, enc);
         }
     }
     if (p > e) p = e;